The aim of this project is to analyze the "genetic transmission" of the mammary tumor virus in mice. Results of testing for MTV antigen in milk of strains GR, and C57BL and their F1, F2, and backcross hybrids suggest segregation of a single dominant gene. Similar results from second backcross females tend to support this conclusion. The occurrence of females with intermediate titres of antigen, however, suggest modifying factors. At present there is no conclusive evidence that this locus carries the viral information. It may be only a susceptibility gene or controller gene that controls replication of viral information located elsewhere. Much work remains to be done to clarify the picture in respect to segregation of antigen and these results must later be correlated with occurrence of mammary tumors.